With the rapid increase in the number of mobile communication terminals in virtually all parts of the world during the last decade, it is not surprising that network operators and other parties including governments and non-governmental organizations have begun providing services that can alert mobile terminal users of emergencies such as natural disasters in the form of earthquakes and tsunamis etc. The general concept is that detections of emergencies are collected at specific detection centres and these detections are then formulated into warning notifications and messages that are distributed via mobile communication network operators to the users of communication terminals that are subscribers in the networks.
Recently, the 3rd generation partnership project (3GPP) has standardized such public warning systems (PWS). For example, the earthquake and tsunami warning system (ETWS), the commercial mobile alert system (CMAS) and the EU-Alert System. These systems use cell broadcast service, CBS, as the underlying technology, in e.g. GSM and WCDMA systems, to broadcast emergency messages to the users. CBS is a 3GPP feature in which a number of short unacknowledged messages can be broadcasted to all mobile communication terminals (UE) in a particular region. After the standardization of these systems, CBS has obtained increased interest from operators, network vendors and UE manufacturers. The earthquake early warning system which is being deployed in Japanese networks also uses CBS for broadcasting emergency messages.
However, a drawback associated with the use of CBS is one of high battery consumption in the mobile communication terminals when the service is enabled. In fact, for a terminal which has CBS enabled, the battery life is reduced by half or more. The battery consumption problem is reduced to a certain extent in the ETWS system since the CBS reception is enabled only after a primary notification (PN) is received. During the periods during which these notifications are transmitted, it is important that the UE stays powered on to receive further emergency CBS messages in the form of so-called secondary notifications (SN).
For example, tests have shown that power consumption for a UE enabled to receive CBS messages with Level 2 discontinuous reception (DRX) is 2.3 mA and 2.75 mA if the repetition period is 40 seconds and 20 seconds, respectively. When CBS is disabled, the UE consumes 1.945 mA. For a UE that utilizes level 1 DRX, the power consumption is significantly higher than that when level 2 DRX is utilized.
Another problem with the current warning systems that use CBS for the secondary notifications is that, after the primary notification has been received, the UE will follow normal cell re-selection rules without taking the aspect of successfully reception of CBS messages into account. This may in some situations, e.g. areas with reduced radio coverage, lead to that it either takes very long time to retrieve secondary notifications or they are not received at all due to too many cyclic redundancy check (CRC) errors.